Highly pressurized, ultra high pure liquid carbon dioxide is required for a variety of industrial processes. For example, some of the evolving applications in the electronic industry demand the use of supercritical carbon dioxide at high flow rates and high pressures. One of the methods for achieving high pressure carbon dioxide has been to pump liquid carbon dioxide to a desired pressure. However, pumping liquid carbon dioxide to a high pressure can introduce impurities, such as particulates, hydrocarbons, halocarbons, etc., to the product stream.
Other applications, such as photo-resist removal, deposition, lithography, etc, require ultra high purity (UHP) liquid carbon dioxide delivered to a point of use (POU) in pressure range from 2,000 psig to 10,000 psig. The latter depends on many factors for example, specifics of an application, tool design, process philosophy, etc.
The following patents found during a patent search are representative of process for delivery of ultra high purity high pressure gases including liquid carbon dioxide. The common feature of these patents is an attempt to create a pump free system to elevate gas pressure while preserving gas purity.
U.S. Pat. No. 6,023,933 discloses a process which is capable of delivering UHP argon at pressures up to 67,000 psig. In the process a high purity gas is provided in a liquefied physical state, introduced to a vaporization vessel, and then, heated in an isochoric vaporizer sufficiently to vaporize the liquefied gas. As the liquid is vaporized in the isochoric vaporizer the pressure builds to the desired pressure, e.g., from 10,000 to 60,000 psig. When the liquid is substantially vaporized, another unit is used for vapor delivery. This process is good for gas delivery but can not be used to deliver liquid phase to the point of use.
U.S. Pat. No. 6,327,872 discloses a process for delivering liquid carbon dioxide to a point of use at pressures of 750 to 1020 psig. The patent acknowledges in the background the production of high-pressure liquid delivery systems up to about 68 bar by means of a pump and the problems associated therewith. Their approach is to deliver liquid carbon dioxide to an accumulation vessel and then heat the liquid carbon dioxide contained therein thereby elevating the pressure. Using this method is possible to elevate liquid carbon dioxide from a bulk supply pressure of approximately 300 psig to a maximum of about 1020 psig. If isochoric heating continues above the critical point of about 310xc2x0 C., liquid carbon dioxide is converted into a supercritical fluid. Thus, the practical limitation of this process as a liquid delivery system is under the carbon dioxide critical pressure of about 1071 psig and at a temperature below 31xc2x0 C. This process is limited by the maximum achievable pressure of liquid carbon dioxide which can be delivered to a point of use. Also, there is no provision for a mechanism for liquid delivery from the heated vessel unless the liquid is pressurized by means of another fluid at higher pressure. The use of different fluid (other then purified carbon dioxide) can potentially lead to delivered product contamination.
Therefore, there is a need to create a system which can deliver UHP high pressure liquid carbon dioxide, e.g., greater than about 1071 psig from a purifier to a point of use (POU) without the use of any mechanical devices.
This invention relates to an improvement in a process and apparatus for delivering high pressure liquid carbon dioxide to a point of use at pressures above ambient pressure without pumping or use of any mechanical devises with moving parts. In the process a high purity carbon dioxide feed in gaseous or preferably in liquid form is charged to a vessel and at least partially solidified. As the liquid is converted to a solid phase, additional liquid is added until the vessel is at least substantially filled. Once filled, the liquid is isochorically heated, i.e., heated at constant volume whereby initialy the solid phase carbon dioxide is partially converted to a liquid until a desired pressure is reached. Liquid, then, is withdrawn from the vessel at a desired pressure at a rate at which the solid phase carbon dioxide is converted to a liquid.
There are significant advantages to the process for delivering liquid carbon dioxide to a point of use and these include:
an ability to avoid the use of pumps or compressors and yet being able to deliver purified carbon dioxide liquid stream to a point of use at virtually any desired pressure;
an ability to deliver a supercritical fluid to the point of use (POU) at the desired pressure with minimal energy input. For example, one can utilize the energy already delivered with liquid carbon dioxide;
an ability to achieve energy efficiency by operating over a narrow temperature range since the solid to liquid melting process occurs at practically constant temperature;
an ability to deliver ultra high purity carbon dioxide to a point of use with simple, noncomplex equipment, e.g., chillers and heaters, without incurring contamination in the process due to complete exclusion of any source of contamination such as devises with moving parts, lubricats, seals, etc.; and,
an ability to deliver liquid at high pressure at essentially room temperature.
The drawing is schematic of a three unit apparatus and process for the delivery of liquid carbon dioxide to a point of use at variable pressure by the isochoric eating of solid carbon dioxide.